Lubricant composition for automatic transmission

ABSTRACT

The invention provides a lubricant composition for automatic transmissions excellent in sustainability of anti-shudder property, low-temperature viscosity characteristics, and oxidation stability, and excellent and well-balanced in durability of friction characteristics, energy-conserving performance, and anti-fatigue performance on gears. The composition contains a base oil having a kinematic viscosity at 100° C. of 3.7-4.1 mm 2 /s, and composed of base oil having the viscosity of 2.5-4.5 mm 2 /s, and base oil having the viscosity of 10-40 mm 2 /s; 1-20 mass % poly(meth)acrylate viscosity index improver having Mw of 15000-300.00; 2-4 mass % imide friction modifier having a C8-C30 hydrocarbon group; 0.01-0.04 mass % phosphorus extreme pressure agent in terms of phosphorus; and 0.01-0.04 mass % ashless dispersant having at least one alkyl or alkenyl group of a number average molecular weight of ≧2000 in terms of nitrogen, all with respect to the total amount of the composition, and has the viscosity of 5.6-5.8 mm 2 /s.

FIELD OF ART

The present invention relates to a lubricant composition for automatictransmissions, in particular, to a lubricant composition for automatictransmissions, particularly for vehicles, which prolongs fatigue life ofgears irrespective of its low viscosity, and which is excellent andwell-balanced in sustainability of anti-shudder property,low-temperature viscosity characteristics, oxidation stability, anddurability of friction characteristics.

BACKGROUND ART

There is recently a pressing need for saving energy consumption ofvehicles, or energy conservation, in order to address environmentalissues, for example, by reduction of CO₂ emission. Engines and automatictransmissions are strongly demanded to contribute to energy-saving, andlubricants therefor are desired to have lower agitation resistance andfriction resistance.

One means for making automatic transmissions energy-conserving is tolower the viscosity of a lubricant used therein. By lowering theviscosity of lubricants used in a vehicle automatic transmission forexample, which has a torque converter, a wet clutch, gear-bearingmechanisms, an oil pump, and a hydraulic control mechanism, theagitation resistance and friction resistance in these parts are reduced,which results in improvement in power transmission efficiency and thusin vehicle fuel efficiency.

However, the lowered viscosity of lubricants used in these parts mayremarkably shorten their fatigue life, causing seizure or the like toresult in troubles in the transmission. In particular, when a phosphorusextreme pressure agent is used for improving extreme pressure propertyof a low-viscosity lubricant, fatigue life of gears is remarkablyimpaired, so that it is usually hard to lower the viscosity of suchlubricant. On the other hand, a sulfur extreme pressure agent mayimprove fatigue life of gears, but impairs oxidation stability, so thata large amount of antioxidants is required.

As a conventional automatic transmission fluid for vehicles capable ofmaintaining various performances, such as transmission characteristics,for a prolonged period of time, there are reported optimized blends ofsynthetic and/or mineral lubricant base oils, anti-wear agent, extremepressure agent, metal detergent, ashless dispersant, friction modifier,viscosity index improver, and the like (JP-3-39399-A, JP-7-268375-A,JP-2000-63869-A, and JP-2001-262176-A).

These compositions, however, are not intended to improve fuelefficiency, and thus have a high kinematic viscosity. No discussion ismade in these publications regarding the effect of reduction inlubricant viscosity on fatigue life of gears. Sufficient discussion hasnot been made on a composition that may solve such a problem. There arerecently proposed automatic transmission fluids having a loweredviscosity, for example, in JP-2004-169025-A, JP-2004-155924-A, andJP-2004-155873-A. These lubricants have excellent sustainability ofanti-shudder property, low-temperature viscosity characteristics, andoxidation stability, but are yet to be improved in excellence andbalance of durability of friction characteristics, contribution toenergy conservation, and anti-fatigue performance on gears.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a lubricantcomposition for automatic transmissions which has excellentsustainability of anti-shudder property, low-temperature viscositycharacteristics, and oxidation stability, and is excellent andwell-balanced in durability of friction characteristics,energy-conserving performance, and anti-fatigue performance on gears, inparticular a lubricant composition for automatic transmissions,particularly suitable for vehicle automatic transmissions, having bothenergy-conserving performance and anti-fatigue performance on gears.

The present inventors have made intensive studies for solving the aboveproblems to find out that a low-viscosity lubricant composition forautomatic transmissions wherein particular base oils and particularadditives are combined, may solve the above problems, thereby completingthe present invention.

According to the present invention, there is provided a lubricantcomposition for automatic transmissions comprising:

(A) a lubricant base oil having a kinematic viscosity at 100° C. of 3.7to 4.1 mm²/s, and consisting of lubricant base oil (A1) having akinematic viscosity at 100° C. of 2.5 to 4.5 mm²/s and lubricant baseoil (A2) having a kinematic viscosity at 100° C. of 10 to 40 mm²/s;

(B) a poly(meth)acrylate viscosity index improver having a weightaverage molecular weight of 15000 to 30000 (sometimes referred to ascomponent (B) hereinbelow) at 1 to 20 mass % of the total amount of thecomposition;

(C) an imide friction modifier having a hydrocarbon group with 8 to 30carbon atoms (sometimes referred to as component (C) hereinbelow) at 2to 4 mass % of the total amount of the composition;

(D) a phosphorus extreme pressure agent (sometimes referred to ascomponent (D) hereinbelow) at 0.01 to 0.04 mass % of the total amount ofthe composition in terms of phosphorus;

(E) an ashless dispersant having at least one alkyl or alkenyl group ofa number average molecular weight of not lower than 2000 (sometimesreferred to as component (E) hereinbelow) at 0.01 to 0.04 mass % of thetotal amount of the composition in terms of nitrogen;

wherein said composition has a kinematic viscosity at 100° C. of 5.6 to5.8 mm²/s (sometimes referred to as the present compositionhereinbelow).

The present composition is capable of prolonging fatigue life of gearsirrespective of its low viscosity, has excellent sustainability ofanti-shudder property, low-temperature viscosity characteristics, andoxidation stability, and is excellent and well-balanced in durability offriction characteristics, energy-conserving performance, andanti-fatigue performance on gears. The present composition isparticularly suitable for vehicle automatic transmissions, has bothenergy-conserving performance and anti-fatigue performance on gears, andis capable of achieving energy conservation of vehicles.

PREFERRED EMBODIMENTS OR THE INVENTION

The present invention will now be explained in detail.

The present composition is characterized in that the particularlubricant base oil (A) and particular components (B) to (E) arecontained at a good balance, and the kinematic viscosity at 100° C. isin the range of 5.6 to 5.8 mm²/s.

Having the particular composition and the kinematic viscosity in theparticular range mentioned above, the present composition is capable ofprolonging fatigue life of gears irrespective of its low viscosity, andis excellent and well-balanced in sustainability of anti-shudderproperty, low-temperature viscosity characteristics, oxidationstability, and durability of friction characteristics.

If the kinematic viscosity at 100° C. of the present composition ishigher than 5.8 mm²/s, the energy-conserving performance given byreduction of agitation resistance and excellent low-temperatureviscosity characteristics cannot be achieved sufficiently. If thekinematic viscosity is lower than 5.6 mm²/s, fatigue life of gearscannot be prolonged sufficiently

According to the present invention, the lubricant base oil (A) includeslubricant base oil (A1) having a kinematic viscosity at 100° C. of 2.5to 4.5 mm²/s (sometimes referred to as component (Al) hereinbelow), i.e.one or more lubricant base oils (A1) selected from the group consistingof mineral and synthetic base oils having the particular kinematicviscosity, and lubricant base oil (A2) having a kinematic viscosity at100°C. of 10 to 40 mm²/s (sometimes referred to as component (A2)hereinbelow).

In component (A1), the mineral base oil may be, for example, paraffin ornaphthene mineral base oils refined by atmospheric-distilling crude oilfollowed by vacuum-distillation of the atmospheric residue, and refiningthe resulting lubricant fraction by one or a suitable combination ofsolvent deasphalting, solvent extraction, hydrocracking, solventdewaxing, contact dewaxing, hydrorefining, washing with sulfuric acid,and clay treatment; or normal paraffin or isoparaffin. One or acombination of two or more of these base oils at any ratio may be used.

Examples of preferred mineral base oils may be as follows:

-   (1) distillate oil obtained by atmospheric distillation of    paraffinic and/or mixed-base crude oils;-   (2) distillate oil obtained by vacuum-distilling the atmospheric    residue of paraffinic and/or mixed-base crude oils (WVGO);-   (3) wax obtained by lubricant dewaxing and/or Fischer-Tropsch wax    produced by GTL process and the like;-   (4) mild-hydrocracked (MHC) oil of one or a mixture of two or more    of above (1) to (3);-   (5) mixed oil of two or more of above (1) to (4);-   (6) deasphalted oil (DAO) of above (1), (2), (3), (4), or (5);-   (7) mild-hydrocracked (MHC) oil of above (6); and-   (8) lubricant obtained by refining a mixed oil of two or more of    above (1) to (7) as a charge stock and/or a lubricant fraction    recovered from this charge stock, by ordinary refining processes,    and recovering the lubricant fraction.

The ordinary refining processes as used herein are not particularlylimited, and any refining processes used in production of lubricant baseoils may be employed. Examples of the ordinary refining processes mayinclude (a) hydrorefining, such as hydrocracking or hydrofinishing; (b)solvent refining, such as furfural solvent extraction; (C) dewaxing,such as solvent dewaxing or contact dewaxing; (d) clay refining usingacid clay or activated clay; and (e) chemical (acid or alkali) refining,such as washing with sulfuric acid or caustic soda. One or anycombination in any order of these refining processes may be employed inthe present invention.

Particularly preferred mineral base oils may be those obtained bysubjecting a base oil selected from above (1) to (B) to the followingtreatment.

That is, hydrocracked mineral oils and/or wax-isomerized isoparaffinbase oils may preferably be used, which are obtained by subjecting thelubricant fraction of a base oil selected from above (1) to (8) tohydrocracking or wax-isomerization, then subjecting the resultingproduct or the lubricant fraction thereof to dewaxing, such as solventor contact dewaxing, followed by solvent refining or to solvent refiningfollowed by dewaxing, such as solvent or contact dewaxing. Thehydrocracked mineral oil and/or wax-isomerized isoparaffiin base oil ispreferably used in an amount of not less than 30 mass %, more preferablynot less than 50 mass %, most preferably not less than 70 mass % of thetotal amount of the mineral base oil.

In component (A1), the synthetic base oil may be, for example,poly-α-olefin or hydrides thereof, isobutene oligomer or hydridesthereof, isoparaffin, alkylbenzene, alkylnaphthalene; diesters, such asditridecyl glutarate, di-2-ethylhexyl adipate, isodecyl adipate,ditridecyl adipate, or di-2-ethylhexyl sebacate; polyol esters, such astrimethylolpropane caprylate, trimethylolpropane pelargonate,pentaerythritol2-ethylhexanoate, or pentaerythritol pelargonate;polyoxyalkylene glycols, dialkyldiphenyl ethers, or polyphenyl ethers.

Among the above synthetic base oils, for example, poly-α-olefin orhydrides thereof may preferably be used. Poly-α-olefin may be anoligomer or a co-oligomer of α-olefin having usually 2 to 32, preferably6 to 16 carbon atoms. More specifically, 1-octene oligomer, 1-deceneoligomer, ethylene-propylene co-oligomer, or hydrides thereof, may beused.

The poly-α-olefin maybe prepared by any process without specificlimitation, for example, by polymerizing α-olefin in the presence of apolymerization catalyst, such as a Friedel-Crafts catalyst, includingaluminum trichloride, boron trifluoride, or a complex of borontrifluoride with water, alcohol, such as ethanol, propanol, or butanol,carboxylic acid, or an ester, such as ethyl acetate or ethyl propionate.

It is sufficient that component (A1) contains at least one mineral orsynthetic base oil. For example, two or more mineral base oils, two ormore synthetic base oils, a mixture thereof, or a mixture of at leastone mineral base oil and at least one synthetic base oil may be used.When component (A1) is a mixture, the mixing ratio of the base oilstherein may be selected arbitrarily, as long as the kinematic viscosityat 100° C. of each base oil is 2.5 to 4.5 mm₂/s.

Component (A1) may preferably be one or more base oils selected from thefollowing base oils (A1a) to (A1c):

-   (A1a) mineral base oils having a kinematic viscosity at 100° C. of    2.5 to 3.5 mm²/s, preferably 2.5 to 3.2 mm²/s;-   (A1b) mineral base oils having a kinematic viscosity at 100° C. of    3.5 to 4.5 mm²/s, preferably 3.8 to 4.3 mm²/s; and-   (A1c) poly-α-olefin base oils having a kinematic viscosity at    100° C. of 2.5 to 4.5 mm²/s, preferably 3.8 to 4.3 mm²/s.

The % C_(A) of component (A1), such as base oils (A1a) to (A1c), is notparticularly limited, and may preferably be not higher than 3, morepreferably not higher than 2, and particularly preferably not higherthan 1. With the % C_(A) of component (A1), or even lubricant base oil(A), of not higher than 3, a composition having still more excellentoxidation stability may be obtained

As used herein, the % C_(A) refers to a percent of the number ofaromatic carbons with respect to the total carbon number determined inaccordance with ASTM D 3238-85.

The viscosity index of component (A1), such as base oils (A1a) to (A1c),is not particularly limited, and may preferably be not lower than 80,more preferably not lower tan 90, particularly preferably not lower than110, and usually not higher than 200, more preferably not higher than160. With the viscosity index of not lower than 80,a compositionexhibiting excellent viscosity characteristics from lower temperaturesto higher temperatures maybe obtained, but with too high a viscosityindex, the effect of the composition on fatigue life of gears may belowered.

The sulfur content of component (A1), such as base oils (A1a) to (A1c),is not particularly limited, and may preferably be not higher than 0.05mass %, more preferably not higher than 0.02 mass %, particularlypreferably not higher than 0.005 mass % of the total amount of component(A1). With a reduced sulfur content of component (A1), a compositionhaving still more excellent oxidation stability may be obtained.

Each of base oils (A1a) to (A1c) may be used alone or mixed in anycombination or ratio. It is particularly preferred to use a combinationof base oil (A1a) and base oil (A1b) and/or (A1c). When a combination ofbase oil (A1a) and/or (A1b) and base oil (A1c) is used, the content ofbase oil (A1c) is preferably 1 to 50 mass %, more preferably 3 to 20mass %, particularly preferably 5 to 15 mass % of the total amount oflubricant base oil (A). With the content of base oil (A1c) being about 5to 15 mass %, a composition having excellent anti-fatigue performance,low-temperature characteristics, and oxidation stability may be obtainedeffectively at low cost.

In lubricant base oil (A), component (A2) acts for further improvingfatigue life of gears, and may be one or more base oils selected fromthe following base oils (A2a) to (A2c):

-   (A2a) mineral and/or synthetic base oils, preferably mineral base    oils, having a kinematic viscosity at 100° C. of 10 to 15 mm²/s,    preferably 10 to 12 mm²/s;-   (A2b) mineral and/or synthetic base oils, preferably mineral base    oils, having a kinematic viscosity at 100° C. of 15 to 25 mm²/s,    preferably 17 to 23 mm²/s; and

(A2c) mineral and/or synthetic base oils, preferably mineral base oils,having a kinematic viscosity at 100° C. of 25 to 40 mm²/s, preferably 28to 35 mm²/s.

The % C_(A) of component (A2), such as base oils (A2a) to (A2c), isusually 0 to 40 but not particularly limited, and may preferably be notlower than 2, more preferably not lower than 4, particularly preferablynot lower than 6, and preferably not higher than 15, more preferably nothigher than 10, particularly preferably not higher than 8, for balancinggood oxidation stability and anti-fatigue performance.

The viscosity index of component (A2), such as base oils (A2a) to (A2c),is not particularly limited, and may preferably be not lower than 80,more preferably not lower than 90, particularly preferably not lowerthan 95, and usually not higher than 200, preferably not higher than120, more preferably not higher than 110, particularly preferably nothigher than 100. With the viscosity index of not lower than 80, acomposition exhibiting excellent viscosity characteristics from lowertemperatures to higher temperatures may be obtained, but with too high aviscosity index, the effect of the composition on fatigue life of gearsmay be lowered.

The sulfur content of component (A2), such as base oils (A2a) to (A2c),is not particularly limited, and may usually be 0 to 2mass %, preferably0.05 to 1.5 mass % more preferably 0.3 to 1.2 mass %, still morepreferably 0.5 to 1 mass %, particularly preferably 0.7 to 1.0 mass % ofthe total amount of component (A2). With component (A2) having arelatively high sulfur content, anti-fatigue performance may beimproved, whereas with component (A2) preferably having a sulfur contentof not higher than 1.0 mass %, a composition having still more excellentoxidation stability may be obtained.

According to the present invention, it is preferred to use, as component(A2), base oil (A2b) or (A2c) for improving anti-fatigue performance,and base oil (A2b) for balancing the oxidation stability andanti-fatigue performance. With base oil (A1c) used as component (A1), acomposition having excellent oxidation stability and low-temperatureviscosity characteristics and anti-fatigue performance, may be obtained.

In lubricant base oil (A), the contents of components (A1) and (A2) arenot particularly limited, and the content of component (A1) maypreferably be 70 to 97 mass %, more preferably 85 to 95 mass %, and thecontent of component (A2) may preferably be 3 to 30 mass %, morepreferably 5 to 15 mass %, of the total amount of lubricant base oil (A)

Lubricant base oil (A) which is composed of components (A1) and (A2),has a kinematic viscosity at 100° C. of 3.7 to 4.1 mm²/s, preferably 3.9to 4.1 mm²/s. At a kinematic viscosity at 100° C. of not higher than 4.1mm²/s, fluid resistance is lowered, so that a lubricant compositionexhibiting still lower friction resistance at lubricating sites may beobtained. A composition having excellent low-temperature viscosity, forexample, a Brookfield viscosity at −40° C. of not higher than 15000mPa·s, may be obtained. On the other hand, at a kinematic viscosity at100° C. of not lower than 3.7 mm²/s, a composition may be obtained whichis capable of forming a sufficient oil film, has still more excellentlubricity and anti-fatigue performance, and exhibits still lower baseoil evaporation loss under high-temperature conditions.

The % C_(A) of lubricant base oil (A) is not particularly limited, andmay preferably be not higher than 3, more preferably not higher than 2,particularly preferably not higher than 1, and preferably not lower than0.1, more preferably not lower than 0.5. With the % C_(A) of lubricantbase oil (A) of not higher than 3, a composition having still moreexcellent oxidation stability may be obtained.

The viscosity index of lubricant base oil (A) is not particularlylimited, and may preferably be not lower than 80, more preferably notlower than 90, particularly preferably not lower than 110. With theviscosity index of not lower than 80, a composition exhibiting excellentviscosity characteristics from lower temperatures to higher temperaturesmay be obtained.

The sulfur content of lubricant base oil (A) is not particularlylimited, and may preferably be 0 to 0.3 mass %, more preferably 0.03 to0.2 mass %, particularly preferably 0.06 to 0.1 mass %. With the sulfurcontent of lubricant base oil (A) in the above range, in particular 0.03to 0.2 mass %, anti-fatigue performance and oxidation stability may bebalanced.

In the present composition, component (B) is a poly(meth)acrylateviscosity index improver having a weight average molecular weight of15000 to 30000, which is obtained by diluting a poly(meth)acrylatecompound with a diluent. The weight average molecular weight of thepoly(meth)acrylate compound may preferably be 17000 to 25000, morepreferably 18000 to 24000, for further improvement in anti-fatigueperformance.

As used herein, the weight average molecular weight means a weightaverage molecular weight measured with 150-C ALC/GPC system manufacturedby WATERS CORPORATION equipped with two GMHHR-M (7.8 mmID×30 cm) columnsmanufactured by TOSOH CORPORATION arranged in series, usingtetrahydrofuran as a solvent, at 23° C. , at a flow rate of 1 mL/min, asample concentration of 1 mass %, sample injection volume of 75 μL, anddetermined with a differential refractive index detector (RI) against acalibration curve obtained from polystyrene standard.

The poly(meth)acrylate in the poly(meth)acrylate compound constitutingcomponent (B) may preferably be those having a structural unitrepresented by the formula (1)

In the formula (1), R¹ stands for a hydrogen atom or a methyl group,preferably a methyl group, and R² stands for a hydrocarbon group having1 to 30 carbon atoms or a group represented by the formula −(R)a-E,wherein R stands for an alkylene group having 1 to 30 carbon atoms, Estands for an amine or heterocyclic residue having 1 to 2 nitrogen atomsand 0 to 2 oxygen atoms, and a is 0 or 1.

Examples of the alkyl group having 1 to 30 carbon atoms represented byR² may include methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl,octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl,hexadecyl, heptadecyl, octadecyl, icosyl, docosyl, tetracosyl,hexacosyl, and octacosyl groups. These alkyl groups may be eitherstraight or branched.

Examples of the alkylene group having 1 to 30 carbon atoms representedby R may include methylene, ethylene, propylene, butylene, pentylene,hexylene, heptylene, octylene, nonylene, decylene, undecylene,dodecylene, tridecylene, tetradecylene, pentadecylene, hexadecylene,heptadecylene, and octadecylene groups. These alkylene groups may beeither straight or branched.

Examples of the amine residue represented by E may includedimethylamino, diethylamino, dipropylamino, dibutylamino, anilino,toluidino, xylidino, acetylamino, and benzoylamino groups. Examples ofthe heterocyclic residue represented by E may include morpholino,pyrrolyl, pyrrolino, pyridyl, methylpyridyl, pyrrolidinyl, piperidinyl,quinonyl, pyrrolidonyl, pyrrolidono, imidazolino, and pyrazino groups.

Examples of the poly(meth) acrylate having a structural unit representedby the formula (1) may include poly(meth)acrylates prepared bypolymerizing or copolymerizing one or more monomers represented by theformula (1a):CH₂═CH(R¹)—C(═O)—OR²  (1a)wherein R¹ and R² are the same as those in the formula (1).

Examples of the monomers represented by the formula (1a) may include thefollowing monomers (Ba) to (Be).

Monomer (Ba) is a (meth) acrylate having an alkyl group with 1 to 4carbon atoms, and may specifically be methyl(meth)acrylate,ethyl(meth)acrylate, n- or i-propyl(meth)acrylate, n-, i-, or sec-butyl(meth) acrylate, with methyl (meth)acrylate being preferred.

Monomer (Bb) is a (meth)acrylate having an alkyl or alkenyl group with 5to 15 carbon atoms, and may specifically be octyl.(meth)acrylate,nonyl(meth)acrylate, decyl(meth) acrylate, undecyl(meth)acrylate,dodecyl(meth)acrylate, tridecyl(methacrylate, tetradecyl(meth)acrylate,pentadecyl(meth)acrylate, octenyl meth)acrylate, nonenyl(meth)acrylate,decenyl(meth)acrylate, undecenyl(meth)acrylate, dodecenyl (meth)acrylate, tridecenyl(meth)acrylate, tetradecenyl(meth)acrylate, orpentadecenyl(meth)acrylate. These may be either straight or branched.(Meth)acrylates mainly containing straight alkyl groups with 12 to 15carbon atoms are preferred.

Monomer (Bc) is a (meth) acrylate having a straight alkyl or alkenylgroup with 16 to 30 carbon atoms, preferably a straight alkyl group with16 to 20 carbon atoms, more preferably a straight alkyl group with 16 or18 carbon atoms. Specific examples of monomer (Bc) may includen-hexadecyl(meth)acrylate, n-octadecyl(meth)acrylate,n-icosyl(meth)acrylate, n-docosyl(meth)acrylate,n-tetracosyl(meth)acrylate, n-hexacosyl meth)acrylate, andn-octacosyl(meth) acrylate, with n-hexadecyl (meth) acrylate andn-octadecyl (meth)acrylate being preferred

Monomer (Bd) is a (meth)acrylate having a branched alkyl or alkenylgroup with 16 to 30 carbon atoms, preferably a branched alkyl group with20 to 28 carbon atoms, more preferably a branched alkyl group with 22 to26 carbon atoms. Specific examples of monomer (Bd) may include branchedhexadecyl(meth)acrylate, branched octadecyl (meth) acrylate, branchedicosyl (meth) acrylate, branched docosyl(meth)acrylate, branchedtetracosyl(methacrylate, branched hexacosyl(meth)acrylate, and branchedoctacosyl(meth)acrylate, (Meth)acrylates represented by the formula—C—C(R³)R⁴, having a branched alkyl group with 16 to 30, preferably 20to 28, more preferably 22 to 26 carbon atoms are preferred. In theformula, R³ and R⁴ are not particularly limited as long as the carbonnumber of C—C—(R³)R⁴ is 16 to 30, and R³ may preferably be a straightalkyl group having 6 to 12, more preferably 10 to 12 carbon atoms, andR⁴ may preferably be a straight alkyl group having 10 to 16, morepreferably 14 to 16 carbon atoms.

Specific examples of monomer (Bd) may include (meth)acrylates having abranched alkyl group with 20 to 30 carbon atoms, such as2-decyl-tetradecyl(meth)acrylate, 2-dodecyl-hexadecyl(meth)acrylate, and2-decyl-tetradecyloxyethyl(meth)acrylate.

Monomer (Be) is a monomer having a polar group. Examples of monomer (Be)may include vinyl monomers having an amido group, monomers having anitro group, vinyl monomers having a primary to tertiary amino group, orvinyl monomers having a nitrogen-containing heterocyclic group;chlorides, nitrides/ or phosphates thereof; lower alkylmonocarboxylates, such as those having 1 to 8 carbon atoms, vinylmonomers having a quaternary ammonium salt group, amphoteric vinylmonomers containing oxygen and nitrogen, monomers having a nitrilegroup, vinyl aliphatic hydrocarbon monomers, vinyl alicyclic hydrocarbonmonomers, vinyl aromatic hydrocarbon monomers, vinyl esters, vinylethers, vinyl ketones, vinyl monomers having an epoxy group, vinylmonomers having a halogen, unsaturated carboxylates, vinyl monomershaving a hydroxyl group, vinyl monomers having a polyoxyalkylene chain,vinyl monomers having an ionic group, such as anionic, phosphate,sulfonate, or sulfate group; monovalent metal salts, divalent metalsalts, amine salts, or ammonium salts thereof.

As monomer (Be), monomers containing nitrogen are preferred among these,which may be, for example, 4-diphenylamine (meth)acrylamide,2-diphenylamine (meth)acrylamide, dimethylaminoethyl (meth)acrylamide,diethylaminoethyl (meth) acrylamide, dimethylaminopropyl(meth)acrylamide, dimethylaminomethyl methacrylate, diethylaminomethylmethacrylate, dimethylaminoethyl (meth)acrylate, diethylaminoethyl(meth)acrylate, morpholinomethyl methacrylate, morpholinoethylmethacrylate, 2-vinyl-5-methylpyridine, or N-vinylpyrrolidone.

Component (B) in the present invention may preferably be a viscosityindex improver containing a poly(meth)acrylate compound obtained bypolymerizing or copolymerizing one or more monomers selected from abovemonomers (Ba) to (Be).

More preferred examples of such poly(meth)acrylate compound may include:

-   1) non-dispersant type poly(meth)acrylate which is a copolymer of    monomers (Ba) and (Bb), or hydrides thereof;-   2) non-dispersant type poly:(meth)acrylate which is a copolymer of    monomers (Ba), (Bb), and (Bc), or hydrides thereof;-   3) non-dispersant type poly(meth)acrylate which is a copolymer of    monomers (Ba), (Bb), (Bc), and (Bd) or hydrides thereof;-   4) dispersant type poly(meth) acrylate which is a copolymer of    monomers (Ba), (Bb), and (Be), or hydrides thereof;-   5) dispersant type poly(meth)acrylate which is a copolymer of    monomers (Ba), (Bb), (Bc), and (Be), or hydrides thereof; and-   6) dispersant type poly(meth)acrylate which is a copolymer of    monomers (Ba), (Bb), (Bc), (ad), and (Be), or hydrides thereof

Among these, non-dispersant type poly(meth)acrylate compounds 1) to 3)above are more preferred, and non-dispersant type poly(meth)acrylatecompounds 2) and 3) are still more preferred, and non-dispersant typepoly (meth) acrylate compound 3) is particularly preferred.

In the present composition, component (B) is usually provided in a statewherein the poly (meth) acrylate compound has been diluted to about 10to 80 mass with a diluent in light of easy handling and solubility inlubricant base oil (A) Thus the content of component (B) including thediluent is 1 to 20 mass %, preferably 2 to :12 mass %, more preferably 3to 8 mass % of the total amount of the composition.

If the content of component (B) is over the above range, improvement inanti-fatigue performance in proportion to the content may not beexpected, shear stability is poor, the initial extreme pressure propertyis hard to be maintained for a prolonged period of time, and the effecton fuel efficiency resulting from the reduction of viscosity may belowered.

The content of component (B) may suitably be selected from the aboverange depending on the kind of the poly(meth)acrylate compound or theratio of the diluent, so that the kinematic viscosity at 100° C. of thepresent composition is 5.6 to 5.8 mm²/s.

In the present composition, component (C) is an imide friction modifierhaving a hydrocarbon group with 8 to 30 carbon atoms.

Component (C) is not particularly limited as long as it is a compoundhaving an imide structure and a hydrocarbon group with 8 to 30 carbonatoms, and may preferably be, for example, a succinimide represented bythe formula (2) or (3) and/or a derivative thereof;

In the formula (2), R⁵ stands for a straight or branched hydrocarbongroup having 8 to 30 carbon atoms, R⁶ stands for a hydrogen atom or astraight or branched hydrocarbon group having 1 to 3 carbon atoms, R⁷stands for a hydrocarbon group having 1 to 4 carbon atoms, and m is aninteger of 1 to 7.

In the formula (3), R⁶ and R⁹ independently stand for a straight orbranched hydrocarbon group having 8 to 30 carbon atoms, R¹⁰ and R¹¹,independently stand for a hydrocarbon group having 1 to 4 carbon atoms,and n is an integer of 1 to 7.

R⁵ in the formula (2) and R⁸ and R⁹ in the formula (3) independentlystand for a straight or branched hydrocarbon group having 8 to 30,preferably 12 to 25 carbon atoms. Examples of such a hydrocarbon groupmay include alkyl and alkenyl groups, with an alkyl group beingpreferred. Examples of the alkyl and alkenyl groups may include octyl,octenyl, nonyl, nonenyl, decyl, decenyl, dodecyl, dodecenyl, octadecyl,and octadecenyl groups, as well as straight or branched alkyl grouphaving up to 30 carbon atoms. If the carbon number of the hydrocarbongroup is less than 8or more than 30, sufficient anti-shudder property ishardly obtained. Thus it is particularly preferable that the hydrocarbongroup is a branched alkyl group having 8 to 30, more preferably 10 to 25carbon atoms. With a branched alkyl group having 8 to 30 carbon atoms,deterioration of torque capacity of various wet clutches may bedecreased compared to the case with a straight alkyl group, and acomposition excellent in both capability of maintaining torque capacityand sustainability of anti-shudder property, may be obtained.

R⁷ in the formula (2) and R¹⁰ and R¹¹ in the formula (3) independentlystand for a hydrocarbon group having 1to 4 carbon atoms. The hydrocarbongroup may be an alkylene group having 1 to 4 carbon atoms, preferably analkylene group having 2 or 3 carbon atoms, such as an ethylene orpropylene group.

R⁶ in the formula (2) may be, for example, a straight or branched alkylor alkenyl group having 1 to 30 carbon atoms, preferably a branchedalkyl or alkenyl group having 1 to 30, more preferably 8 to 30, andparticularly preferably 10 to 25 carbon atoms, with the branched alkylgroup being particularly preferred.

In the formulae (2) and (3), n and m each denote an integer of 1 to 7,For obtaining a composition with still higher sustainability ofanti-shudder property, n and m each preferably denote 1, 2, or 3,particularly preferably

The succinimide compound represented by the formula (2) or (3) may beprepared by a known method, for example, by reacting an alkyl or alkenylsuccinic anhydride and polyamine. Specifically, monosuccinimiderepresented by the formula (2) wherein R⁶is a hydrogen atom may beprepared, for example, by gradually adding dropwise 1 mole of succinicanhydride having a straight or branched alkyl or alkenyl group with 8 to30 carbon atoms to I mole or more of polyamine, such asdiethylenetriamine, triethylenetetramine, or tetraethylenepentamine, ina nitrogen atmosphere at 130 to 180° C. , preferably 140 to 175° C. ,allowing to react for 1 to 10 hours, preferably 2 to 6 hours, anddistilling off the unreacted polyamine. Monosuccinimide represented bythe formula (2) wherein R⁶ is a hydrocarbon group having 1 to 30 carbonatoms may be prepared, for example, by reactingN-octadecyl-1,3-propanediamine and the succinic anhydride mentionedabove in the same way as outlined above. Bissuccinimide represented bythe formula (3) may be prepared by adding dropwise 0.5 mole of thepolyamine mentioned above to 1 mole of the succinic anhydride mentionedabove under the same conditions as outlined above, allowing to react inthe same way, and evaporating the generated moisture.

Examples of the derivatives of the succinimide represented by theformula (2) or (3) may include compounds resulting from modification ofthe succinimide with boric acid, phosphoric acid, carboxylic acid, orderivatives thereof, sulfuric compounds, or triazoles. Specific examplesof and methods for producing the derivatives may be those specificallydisclosed in JP-2002-105478-A.

In the present invention, as component (C), use of bis-type succinimiderepresented by the formula (3) is particularly preferred compared to useof mono-type succinimide represented by the formula (2), for itscapability of giving higher sustainability of anti-shudder property tothe composition.

In the present composition, the content of component (C) is 2 to 4 mass%, preferably 2.5 to 3.5 mass % of the total amount of the composition.At a content of less than 2 mass %, the sustainability of anti-shudderproperty may not be made to achieve the higher goal of the presentinvention, for example, 1000 hour or longer life of anti-shudderproperty, whereas at a content of more than 4 mass %, anti-fatigueperformance may be impaired.

Component (D) in the present composition is a phosphorus extremepressure agent. Specific examples of component (D) may includemonophosphates, diphosphates, and triphosphates, monophosphites,diphosphites, triphosphites, having an alkyl or aryl group with 3 to 30,preferably 4 to 18 carbon atoms, and amine or alkanolamine saltsthereof. Among these, phosphates and phosphites having an alkyl groupwith 3 to 30 carbon atoms are preferred, and phosphites having an alkylgroup with 3 to 30 carbon atoms are particularly preferred.

In the present composition, the content of component (D) is 0.01 to 0.04mass %, preferably 0.02 to 0.04 mass % of the total amount of thecomposition in terms of phosphorus. If the content of component (D) interms of phosphorus is less than the above range, sustainability ofanti-shudder property tends to be lowered, whereas if the content isover the above range, anti-fatigue performance tends to be impaired.

In the present composition, component (E) is an ashless dispersanthaving at least one alkyl or alkenyl group of a number average molecularweight of not lower than 2000.

Typical examples of component (E) may include succinimide, benzylamine,or polyamine ashless dispersants having at least one alkyl or alkenylgroup of a number average molecular weight of not lower than 2000. Amongthese, the succinimide ashless dispersants having at least one alkyl oralkenyl group are preferred, and bis-type succinimide ashlessdispersants having at least two alkyl or alkenyl groups are particularlypreferred.

The number average molecular weight of the alkyl or alkenyl group maypreferably be 2000 to 5000, more preferably 2100 to 3500, still morepreferably 2200 to 3000. The alkyl or alkenyl group may be eitherstraight or branched. A branched alkyl or alkenyl group derived fromoligomer of olefins, such as propylene, 1-butene, or isobutene, orco-oligomer of ethylene and propylene, is preferred, and a polybutenylgroup derived from poly(iso)butene is particularly preferred.

With the number average molecular weight of the alkyl or alkenyl groupbeing not lower than 2000, anti-shudder property may be improved, anddurability of friction characteristics and anti-fatigue performance ongears may be improved. With the number average molecular weight of nothigher than 5000, a compound having still more excellent low-temperaturecharacteristics may easily be obtained.

Specific examples of the succinimide having at least one alkyl oralkenyl group of a :number average molecular weight of not lower than2000 may include compounds represented by the formula (4-a) or (4-b):

In the formula, R¹², R¹³, and R¹⁴ independently stand for an alkyl oralkenyl group having a number average molecular weight of not lower than2000, preferably 2000 to 5000, more preferably a poly(iso)butenyl group;r is an integer of 1to 5, preferably 2 to 4; and s is an integer of 0 to4, preferably 1 to 3.

The above-mentioned succinimide includes so-called monosuccinimide,srepresented by the formula (4-a) wherein succinic anhydride is added toone end of polyamine, and so-called bissuccinimides represented by theformula (4-b) wherein succinic anhydride is added to both ends ofpolyamine. In the present composition, either one of or a mixture ofthese succinimides may be contained, and bis-type succinimides composedmainly of the bissuccinimides are particularly preferred.

The succinimide may be prepared by any process without particularlimitation, for example, by reacting polybutene or polyisobutylenehaving a number average molecular weight of not lower than 2000 withmaleic anhydride at 100 to 200 °C., and reacting the resultingpoly(iso)butenyl succinate with polyamine. The polyamine may be, forexample, diethylenetriamine, triethylenetetramine,tetraethylenepentamine, or pentaethylenehexamine.

Specific examples of benzylamine having at least one alkyl or alkenylgroup of a number average molecular weight of not lower than 2000 mayinclude compounds represented by the formula (4-c):

In the formula, R¹⁵ stands for an alkyl or alkenyl group having a numberaverage molecular weight of not lower than 2000, preferably 2000 to5000, more preferably a poly(iso)butenyl group, and t is an integer of 1to 5, preferably 2 to 4.

The benzylamine represented by the formula (4-c) may be prepared by anyprocess without particular limitation, for example, by reactingpolyolefin, such as propylene oligomer, polybutene, or ethylene-α-olefincopolymer, with phenol to give alkylphenol, which is then reacted withformaldehyde and polyamine, such as diethylenetriamine,triethylenetetramine, tetraethylenepentamine, or pentaethylenehexamine,by the Mannich reaction or the like.

specific examples of the polyamine having at least one alkyl or alkenylgroup with a number average molecular weight of not lower than 2000 mayinclude compounds represented is by the formula (4-d):R¹⁶—NH—(CH₂CH₂NH)q—H  (4-d)

In the formula, R¹⁶ stands for an alkyl or alkenyl group having a numberaverage molecular weight of not lower than 2000, preferably 2000 to5000, more preferably a poly(iso)butenyl group, and q is an integer of 1to 5, preferably 2 to 4.

The polyamine represented by the formula (4-d) may be prepared by anyprocess without particular limitation, for example, by chlorinatingpolyolefin, such as propylene oligomer, polybutene, or ethylene-α-olefincopolymer, and reacting with ammonia or polyamine, such asethylenediamine, diethylenetriamine, triethylenetetramine,tetraethylenepentamine, or pentaethylenehexamine.

Component (E) also includes derivatives of the above nitrogen-containingcompounds, such as succinimide, benzylamine, or polyamine. Examples ofsuch derivatives may include so-called acid-modified compounds obtainedby reacting, to the nitrogen-containing compounds, monocarboxylic acidhaving 2 to 30 carbon atoms, such as aliphatic acid, polycarboxylic acidhaving 2 to 30 carbon atoms, such as oxalic, phthalic, trimellitic, orpyromellitic acid, or anhydrides or esters thereof, alkylene oxidehaving 2 to 6 carbon atoms, or hydroxy(poly)oxyalkylenecarbonate or thelike, to neutralize or amidify all or part of the residual amino and/orimino groups; so-called boron-modified compounds is obtained byreacting, to the nitrogen-containing compounds, boron compounds, such asboric acid, borate salts, or boronic esters, to neutralize or amidifyall or part of the residual amino and/or imino groups; sulfur-modifiedcompounds obtained by reacting a sulfur compound to thenitrogen-containing compounds; and modified compounds obtained bymodifying the nitrogen-containing compounds by a combination of two ormore modifications selected from the acid-, boron-, andsulfur-modifications.

Among the derivatives discussed above, boron-modified compounds ofalkyl- or alkenyl succinimide having a number average molecular weightof not lower than 2000 give finest anti-fatigue performance on gears, sothat it is particularly preferred that component (E) contains suchboron-modified compounds as an essential component.

The mass ratio of boron to nitrogen (B/N ratio) of the boron-modifiedcompounds of the nitrogen-containing compounds is not particularlylimited, and may preferably be not lower than 0.1, more preferably notlower than 0.2, and preferably not higher than 0.6, more preferably nothigher than 0.3. With a boron-modified compound having a B/N ratiowithin the above range, a compound having excellent anti-fatigueperformance on gears may be obtained.

The nitrogen content of component (E) is arbitrary, and may usually be0.01 to 10 mass %, preferably 0.1 to 3 mass %, particularly preferably0.2 to 1 mass %, in light of abrasion resistance, oxidation stability,and friction characteristics.

In the present composition, the minimum content of component (E) is notlower than 0.1 mass %, preferably 0.02 mass % of the total amount of thecomposition in terms of nitrogen, while the maximum content is nothigher than 0.04 mass %, preferably 0.035 mass % of the total amount ofthe composition in terms of nitrogen.

If the content of component (E) is less than 0.01 mass %, durability offriction characteristics and torque capacity are hard to be maintained,and oxidation stability tends to be deteriorated. Even if the content ismore than 0.04 mass %, sufficient effect in proportion to the content isnot achieved, and low-temperature fluidity of the composition oranti-fatigue performance on gears may be deteriorated, thus not beingpreferred.

When a boron-modified compound of the nitrogen-containing compound isessentially contained as component (E), the minimum content thereof isnot lower than 0.003 mass %, preferably not lower than 0.004 mass % ofthe total amount of the composition in terms of boron, while the maximumcontent thereof is not higher than 0.01 mass %, preferably not higherthan 0.008 mass % of the total amount of the composition in terms ofboron.

With the content of the boron-modified compound as component (E) withinthe above-mentioned preferred range in terms of boron, durability offriction characteristics, is torque capacity, low-temperature fluidity,and anti-fatigue performance on gears maybe maintained at high levels ingood balance.

The viscosity index of the present composition is preferably 95 to 200,more preferably 150 to 180 for excellent viscosity-temperaturecharacteristics. Further, a suitable kinematic viscosity at 40° C. ofthe composition is 25 to 30 mm²/s.

The present composition, in order to further improve its performance, orto impart performances necessary as a lubricant oil for automatictransmissions, may optionally contain various additives, such asviscosity index improvers, friction modifiers other than component (C),extreme pressure agents other than component (D), dispersants other thancomponent (E), metal detergents, antioxidants, corrosion inhibitors,rust inhibitors, demulsifiers, metal deactivators, pour pointdepressants, seal swelling agents, foam inhibitors, and coloring agents,alone or in combination, as necessary.

Examples of the viscosity index improver may include knownnon-dispersant or dispersant type polymethacrylates (other thancomponent (B)), non-dispersant or dispersant type ethylene-a-olefincopolymers or hydrides thereof, polyisobutylene or hydrides thereof,styrene-diene hydrogenated copolymers, styrene-maleic anhydride estercopolymers, and polyalkylstyrenes.

The content of the viscosity index improver other than component (B), ifcontained, in the present composition is not particularly limited aslong as the conditions of the kinematic viscosity at 100 °C. of thecomposition is fulfilled, and may usually be 0.1 to 15 mass %,preferably 0.5 to 5 mass % of the total amount of the composition.

The friction modifier other than component (C) may be any compoundusually used as a friction modifier for a lubricant, and may preferablybe, for example, amine compounds, fatty acids, fatty acid esters, fattyacid amides, or fatty acid metal salts, having in their molecule atleast one alkyl or alkenyl group having 6 to 30 carbon atoms, preferablyat least one straight alkyl or alkenyl group having S to 30 carbonatoms.

In the present invention, any one or more compounds selected from thefriction modifiers mentioned above may be contained at any content,which may usually be 0.01 to 5.0 mass %, preferably 0.03 to 3.0 mass %of the total amount of the composition.

As an extreme pressure agent other than component (D), it is preferredto add an extreme pressure agent composed of at least one sulfur extremepressure agent selected from the group consisting of sulfurized oils andfats, sulfurized olefins, dihydrocarbyl polysulfides, dithiocarbamates,thiadiazoles, and benzothiazoles, and/or at least one phosphorus-sulfurextreme pressure agent selected from the group consisting ofthiophosphorous acids, thiophosphorous monoesters, thiophosphorousdiesters, thiophosphorous triesters, dithiophosphorous acids,dithiophosphorous monoesters, dithiophosphorous diesters,dithiophosphorous triesters, trithiophosphorous acids,trithiophosphorous monoesters, trithiophosphorous diesters,trithiophosphorous triesters, and salts thereof.

The content of the extreme pressure agent other than component (D), ifcontained, in the present composition may suitably be selected dependingon its kind.

The dispersant other than component (E) maybe an ashless dispersant,such as succinimide, benzylamine, polyamine, and/or boron compoundderivatives thereof, having a hydrocarbon group with 40 to 400 carbonatoms, other than component (E).

In the present invention, any one or more compounds selected from thedispersants mentioned above may be contained at any content, which mayusually be 0.01 to 15 mass %, preferably 0.1 to 8 mass % of the totalamount of the composition.

Examples of the metal detergents may include alkaline earth metalsulfonates, alkaline earth metal phenates, and alkaline earth metalsalicylates.

In the present invention, any one or more compounds selected from themetal detergents mentioned above may be contained at any content, whichmay usually be 0.01 to 10 mass %, preferably 0.1 to 5 mass % of thetotal amount of the composition.

The antioxidants may be those commonly used for lubricants, such asphenol or amine compounds.

Specific examples of the antioxidants may include alkylphenols, such as2,6-di-tert-butyl-4-methylphenol, bisphenols, such asmethylene-4,4-bisphenol(2,6di-tert-butyl -4-methylphenol),naphthylamines, such as phenyl-α-naphthylamine, dialkyldiphenylamines,zinc dialkyl dithiophosphates, such as zinc di-2-ethylhexyldithiophosphate, and esters of (3,5-di-tert-butyl-4-hydroxyphenyl) fattyacid (such as propionic acid) or (3-methyl-5-tert-butyl-4-hydroxyphenyl)fatty acid (such as propionic acid) and monohydric or polyhydricalcohols, such as methanol, octanol, octadecanol, 1,6-hexadiol,neopentyl glycol, thiodiethylene glycol, triethylene glycol, orpentaerythritol.

Any one or more of the compounds selected from the above may becontained at any content, which may usually be 0.0.1 to 5 mass %,preferably 0.1 to 3 mass % of the total amount of the composition.

Examples of the corrosion inhibitor may include benzotriazol,tolyltriazole, thiadiazole, and imidazole compounds.

Examples of the rust inhibitor may include petroleum sulfonates,alkylbenzene sulfonates, dinonylnaphthalene sulfonates, alkenylsuccinates, and esters of polyhydric alcohols.

Examples of the demulsifier may include polyalkylene glycol nonionicsurfactants, such as polyoxyethylene alkyl ethers, polyoxyethylenealkylphenyl ethers, and polyoxyethylene alkylnaphthyl ethers.

Examples of the metal deactivator may include imidazoline, pyrimidinederivatives, alkylthiadiazoles, mercaptobenzothiazoles, benzotriazolesor derivatives thereof, 1,3,4-thiadiazolepolysulfides,1,3,4-thiadiazolyl-2,5-bisdialkyldithiocarbamates2-(alkyldithio)benzoimidazoles, andβ-(o-carboxybenzylthio.)propionitrile.

The pour point depressant may be selected from known pour pointdepressants depending on the lubricant base oil, and may preferably bepolymethacrylates having a weight average molecular weight of 20000 to500000, more preferably 50000 to 300000, particularly preferably 80000to 200000.

The foam inhibitor may be any compound usually used as a foam inhibitorfor lubricants, for example, silicones, such as dimethyl silicon orfluorosilicon.

The seal swelling agent may be any compound usually used as a sealswelling agent for lubricants, for example, ester, sulfur, or aromaticseal swelling agents.

The coloring agent may be any compound usually used, and may becontained at any content, which may usually be 0.001 to 1.0 mass % ofthe total amount of the composition.

The contents of the above additives, if contained, in the presentcomposition are: 0.005 to 5 mass % for the corrosion inhibitors:, rustinhibitors, or demulsifiers; 0.005 to 2 mass % for the pour pointdepressants or metal deactivators; 0.01 to 5 mass % for the sealswelling agents; and 0.0005 to 1 mass % for the foam inhibitors.

EXAMPLES

The present invention will now be explained in detail with reference toExamples and Comparative Examples, which are illustrative only and donot intend to limit the present invention.

Examples 1-3 and Comparative Examples 1-9

Lubricant compositions for automatic transmissions of the presentinvention (Examples 1 to 3) were prepared with the compositions shown inTable 1. The following performance evaluation tests were conducted onthese compositions. The results are also shown in Table 1.

Lubricant compositions for automatic transmissions for comparison(Comparative Examples 1 to 9) were also prepared with the compositionsshown in Table 1. The same performance evaluation tests were conductedon these compositions. The results are-shown in Table 1.

[Sustainability of Anti-shudder Property]

In accordance with “Test Procedure of Anti-shudder Property of AutomaticTransmission Fluids” provided in JASO M349-98, the low velocity frictiontest was conducted at an oil temperature of 120° C. during thedurability test to evaluate the life of the anti-shudder property of thecompositions of Examples and Comparative Examples. Incidentally, thelife of the reference oil provided in this test procedure is 72 hours,so that the life of the anti-shudder property equivalent to or longerthan this is determined to be excellent. However, the present inventionaimed at 1000 hour or longer life, and after 1500 hours, the test wasdiscontinued.

[Low-Temperature Viscosity Measurement]

In accordance with “Test Procedure of Low-Temperature Viscosity of GearOils” provided in JPI-5S-26-85, the low-temperature viscosities at −40°C. of the lubricant compositions for automatic transmissions weremeasured using a low-temperature oil bath. The present invention aimedat a viscosity of not higher than 15000 mPa·s, but a viscosity of notlower than 10000 mPa·s was found to be desirable for good anti-fatigueperformance.

[Oxidation Stability]

The test oils were subjected to forced degradation by ISOT test (150° C., 96 hours) in accordance with JIS K 2514, and the increase in acidnumber (mgKOH/g) was measured.

[SAE No. 2 Test]

Using SAE No. 2 test machine, durability of transmission characteristicsof a wet clutch was evaluated in accordance with JASO M348-95 “TestProcedure of Friction Characteristics of Automatic Transmission Fluids”except for the following severe test conditions.

<Test Conditions>

Oil Temperature: 120° C.; Revolution: 3000 rpm; Inertial Mass: 0.5kg-m²; Contact Pressure: 1.9 MPa

Only the kinematic friction test was conducted. The clutch was idled at3000 rpm with an inertial mass of 0.5 kg·m², and then pressed withapplied pressure to stop the rotation. The friction coefficient wascalculated from the torque generated at a relative revolution of 1200rpm of the clutch, and recorded.

<Criteria of Evaluation>

The cycle number at which the kinematic friction coefficient wasdecreased for 0.02 or more from the average kinematic frictioncoefficient of 1 to 100 cycles was taken as a durable life cycle of eachcomposition. The durability was evaluated as extremely excellent at 8000cycles or more, and the test was discontinued at 12000 cycles. [GearFatigue Life Test on Actual Device]

Using a commercial 3-speed automatic transmission (AY2 unit)manufactured by JATCO LTD., fatigue life (pitching life) of thereduction gear under loading by a motoring device was evaluated.

<Test Conditions>

Oil Temperature: 120° C.; Revolution: 1000 rpm; Load Torque: 422 N(output shaft torque); Gear: set at 1st position

<Criteria of Evaluation>

The transmission was overhauled every 1000000 cycles, and the cyclenumber at which pitching occurred was taken as the life.

From Table 1, it is understood that the lubricant compositions forautomatic transmissions according to the present invention (Examples 1to 3) containing the particular amounts of components (A) to (E) of thepresent invention provided long fatigue life irrespective of their lowviscosity, exhibited excellent sustainability of anti-shudder property,low-temperature viscosity characteristics, and oxidation stability, andwere excellent and balanced in durability of friction characteristics,energy-conserving performance, and anti-fatigue performance on gears. Onthe other hand, it is understood that when the components defined in thepresent invention were not contained in good balance, any one or more ofthe above performances were not satisfactory (Comparative Examples 1 to9). TABLE 1 Example Comparative Example 1 2 3 1 2 3 Base Oil (A) (basedon total amount of base oil) Base oil (A1a)¹⁾ (mass %) 30 30 40 50 20 30Base oil (A1b)²⁾ (mass %) 60 50 50 40 70 60 Base oil (A1c)³⁾ (mass %) —10 Base oil (A2a)⁴⁾ (mass %) 10 10 10 10 10 10 Base Oil PropertyKinematic Viscosity at 100° C. (mm²/s) 4.0 4.0 3.8 3.6 4.2 4.0 Additives(based on total amount of composition) (B) VM-A10⁵⁾ (mass %) 5 5 6 7 3.8— VM-B11⁶⁾ (mass %) — — — — — 5 (C) Imide FM⁷⁾ (mass %) 3 3 3 3 3 3 (D)Phosporus Extreme Pressure Agent⁸⁾ 0.03 0.03 0.03 0.03 0.03 0.03 ((P)mass %) Phosphorus-Sulfur Extreme Pressure Agent⁹⁾ — — — — — — ((P) mass%) (E) Ashless Dispersant¹⁰⁾ (mass %) 3 3 3 3 3 3 Ashless Dispersant — —— — — — (low-molecular weight mono-type)¹¹⁾ (mass %) N content (mass %)0.027 0.027 0.027 0.027 0.027 0.027 Additive Package¹²⁾ (mass %) 5 5 5 55 5 Composition Properties and Test Results Kinematic viscosity at 100°C. (mm²/s) 5.7 5.7 5.7 5.7 5.7 5.7 Viscosity index 162 164 166 168 157165 Anti-shudder life (h) 1500 1500 1500 1500 1500 1500 Low-temperatureviscosity 14800 12300 13500 12400 18800 13500 (BF method: −40° C.) (mPa· s) Increase in acid number −0.11 −0.13 −0.11 −0.12 0.11 −0.12 (ISOT150° C., after 96 hrs) (mgKOH/g) Durable cycle number of frictioncharacteristics 12000 12000 12000 12000 12000 12000 in SAE No. 2 testGear fatigue test on actual device 3000 3000 3000 2000 3000 2000(thousand cycles) Comparative Example 4 5 6 7 8 9 Base Oil (A) (based ontotal amount of base oil) Base oil (A1a)¹⁾ (mass %) 5 30 30 30 30 30Base oil (A1b)²⁾ (mass %) 95 60 60 60 60 60 Base oil (A1c)³⁾ (mass %)Base oil (A2a)⁴⁾ (mass %) 10 10 10 10 10 Base Oil Property KinematicViscosity at 100° C. (mm²/s) 4.0 4.0 4.0 4.0 4.0 4.0 Additives (based ontotal amount of composition) (B) VM-A10⁵⁾ (mass %) 5 5.2 5.1 5 5.8 3VM-B11⁶⁾ (mass %) — — — — — — (C) Imide FM⁷⁾ (mass %) 3 — 3 3 3 3 (D)Phosporus Extreme Pressure Agent⁸⁾ 0.03 0.03 0.03 — 0.03 0.03 ((P) mass%) Phosphorus-Sulfur Extreme Pressure Agent⁹⁾ — — — 0.03 — — ((P) mass%) (E) Ashless Dispersant¹⁰⁾ (mass %) 3 3 — 3 1 6 Ashless Dispersant — —3 — — — (low-molecular weight mono-type)¹¹⁾ (mass %) N content (mass %)0.027 0.027 0.063 0.027 0.009 0.054 Additive Package¹²⁾ (mass %) 5 5 5 55 5 Composition Properties and Test Results Kinematic viscosity at 100°C. (mm²/s) 5.7 5.7 5.7 5.7 5.7 5.7 Viscosity index 161 163 161 162 164155 Anti-shudder life (h) 1500 24 1200 120 1500 1200 Low-temperatureviscosity 15300 13700 16400 15000 13800 22300 (BF method: −40° C.) (mPa· s) Increase in acid number −0.12 −0.14 −0.03 0.15 0.02 −0.02 (ISOT150° C., after 96 hrs) (mgKOH/g) Durable cycle number of frictioncharacteristics 12000 4000 6000 4000 4000 12000 in SAE No. 2 test Gearfatigue test on actual device 2000 5000 2000 2000 3000 2000 (thousandcycles)Note for Table 1:¹⁾Hydrocracked mineral oil (kinematic viscosity at 100° C.: 2.6 mm²/s; %C_(A): 0; sulfur content: <0.001 mass %; viscosity index: 105)²⁾Hydrocracked mineral oil (kinematic viscosity at 100° C.: 4.2 mm²/s; %C_(A): 0; sulfur content: <0.001 mass %; viscosity index: 125)³⁾Poly-α-olefin base oil (kinematic viscosity at 100° C.: 4.0 mm²/s; %C_(A): 0; sulfur content: 0; viscosity index: 124)⁴⁾Solvent-refined mineral oil (kinematic viscosity at 100° C.: 21.9mm²/s; % C_(A): 7; sulfur content: 0.91 mass %; viscosity index: 95)⁵⁾Non-dispersant type polymethacrylate additive (Mw: 20000)⁶⁾Non-dispersant type polymethacrylate additive (Mw: 50000)⁷⁾Imide friction modifier having C18 hydrocarbon group⁸⁾Alkylphosphite⁹⁾ZnDTP¹⁰⁾Polybutenyl succinimide ashless dispersant (molecular weight ofpolybutenyl group Mw: 2500; bis-type; N content: 0.9 mass %, B content:0.2 mass %)¹¹⁾Polybutenyl succinimide ashless dispersant (molecular weight ofpolybutenyl group Mw: 1300; mono-type; N content: 2.1 mass %)¹²⁾Metal detergent, anti-oxidant, viscosity index improver, dispersant,demulsifier, foam inhibitor, etc.

1. A lubricant composition for automatic transmissions comprising: (A) alubricant base oil having a kinematic viscosity at 100° C. of 3.7 to 4.1mm²/s, and consisting of: lubricant base oil (A1) having a kinematicviscosity at 100° C. of 2.5 to 4.5 mm²/s, and lubricant base oil (A2)having a kinematic viscosity at 100° C. of 10 to 40 mm²/s; (B) apoly(meth)acrylate viscosity index improver having a weight averagemolecular weight of 15000 to 30000 at 1 to 20 mass % of the total amountof the composition; (C) an imide friction modifier having a hydrocarbongroup with 8 to 30 carbon atoms at 2 to 4 mass % of the total amount ofthe composition; (D) a phosphorus extreme pressure agent at 0.01 to 0.04mass % of the total amount of the composition in terms of phosphorus;and (E) an ashless dispersant having at least one alkyl or alkenyl groupof a number average molecular weight of not lower than 2000 at 0.01 to0.04 mass % of the total amount of the composition in terms of nitrogen;wherein said composition has a kinematic viscosity at 100 °C. of 5.6 to5.8 mm²/s.
 2. A method for lubricating an automatic transmissioncomprising the step of lubricating an automatic transmission with alubricant composition comprising: (A) a lubricant base oil having akinematic viscosity at 100° C. of 3.7 to 4.1 mm²/s, and consisting of:lubricant base oil (A1) having a kinematic viscosity at 100° C. of 2.5to 4.5 mm²/s, and lubricant base oil (A2) having a kinematic viscosityat 100° C. of 10 to 40 mm²/s; (B) a poly(meth)acrylate viscosity indeximprover having a weight average molecular weight of 15000 to 30000 at 1to 20 mass % of the total amount of the composition; (C) an imidefriction modifier having a hydrocarbon group with 8 to 30 carbon atomsat 2 to 4 mass of the total amount of the composition; (D) a phosphorusextreme pressure agent at 0.01 to 0.04 mass % of the total amount of thecomposition in terms of phosphorus; and (E) an ashless dispersant havingat least one alkyl or alkenyl group of a number average molecular weightof not lower than 2000 at 0.01 to 0.04 mass % of the total amount of thecomposition in terms of nitrogen; wherein said composition has akinematic viscosity at 100° C. of 5.6 to 5.8 mm²/s.